Hill, Brodrick, and Rule — The Mitchelstoivn Caves. 259 



chemical action on the rock, and the process of excavation along the plane 

 then proceeds at a comparatively rapid rate. 



Erom a consideration of the physical geography and present surface- 

 drainage of the district, it seems improbable that any post-glacial stream of 

 sufficient magnitude to have great erosive power can have flowed through 

 the cave — at any rate through its upper passages. "We are thus driven to the 

 eonclusion that the caves are either glacial or pre-glacial in their formation. 

 During, and especially towards the end of, the glacial period there would 

 naturally be enormous torrents of water ; and thus it is not improbable that 

 these caves, as they now exist, were formed by the glacial streams at the 

 close of that period. 



The first lines of weakness to be considered are the bedding-planes, which, 

 as has been mentioned earlier, dip at angles varying from 30° to 40°, and are 

 the most important factors in the formation of the largest chambers of the 

 caverns. The great East Chamber of the Old Cave is the most remarkable 

 example of the bedding-cave type, and deserves considerable mention. Its 

 floor consists of smooth rock tilted at an angle averaging 35°. Its roof is 

 formed of a bed of similar rock, which dips to within two or three feet of the 

 floor (fig. 1). The bottom of this chamber is filled with clear water, blocked 



B.Stelsctite Pil 



Fig. 1. 

 Section of East Chamber, Old Cave. 



in a few places with masses of fallen rock and clay. There was, at the time of our 

 visit, no sign of flow in this water. The floor and the roof could be seen running 

 down below the water-level, about 3 feet apart ; but it was unfortunately 

 impossible to ascertain the depth of the water owing to the difficulties of 

 the position. The dip of the floor remains constant from the lowest to the 

 highest point, a distance of nearly 200 feet ; but the height of the roof increases 

 considerably, so that at the upper portion of the chamber it is at least 40 feet 

 above the floor. Owing to the difficulty of illuminating such a vast chamber, 



R.I.A. PROC, VOL. XXVII., SECT. B. [2 Q] 



